The invention relates to a gas detection system.
Gas detection systems, where a plurality of sensors detecting the presence of gases and their respective quantities are connected to a common central evaluation system, have been known in many different, though complicated and complex, forms.
For example, a system suited for monitoring larger garages or tunnels is designed in such a manner that gas sampling heads are arranged at different points and connected to a common evaluation center via mechanical hose connections the length of which may reach several hundred meters and even more. The gas mixture is supplied from different sampling points to the sensor in the evaluation center for analysis by switching over cyclically from one hose to the next, which action is usually effected by means of solenoid valves. This system uses of course only one sensor. Apart from the enormous mechanical input required not only for the initial installation, but also for the subsequent monitoring operations, for example with a view to preventing bending or breaking of certain hoses, such a measuring method is also particularly lengthy because important dead times result from the fact that the hoses must be scavenged every time before the gas sensor can be supplied with new gas to be measured. Such a system requires a great number of mechanically moving parts, particularly high performance of its pumps, frequent servicing operations, and all these circumstances are connected with the disadvantage that no redundancy of measurements is obtained, that any self-monitoring of the system is excluded and that the cycle times may be in the range of 30 minutes and above.
One has therefore built up a gas detection system in such a manner that each of the points at which monitoring operations or measurements are to be effected is equipped with a measuring head provided with a sensor and that the measuring heads are connected to an electronic evaluation system via a 5-wire line, under certain circumstances only via a 3-wire line. In the case of a 5-wire line, two of the wires serve for firing the sensor, or for supplying it with current, two other wires serve for transmitting the measured value from the measuring head to the electronic evaluation system, and a fifth wire serves as connection to earth. In the case of a 3-wire line, the earth wire performs simultaneously the function of one line of each of the two other pairs of lines. If another gas analysis is required at the same point or, as usually intended in the case of such systems, at a different point, identical parallel systems, comprising a measuring head and a separate connection cable, must be guided to the electronic evaluation system for detecting the second gas component or gas mixtures at different points, which means that the cost of the system and its installation is quite considerable.
In addition, it is a problem of such known gas detection systems that it is not possible to combine different types of equipment because of the different supply voltages for the measuring heads, different sensitivities and connections. In addition, these systems require separate balancing of the electronic evaluation system up to the measuring head, after assembly and installation of each head, and lengthy calibration operations which will be described hereafter in more detail.
Certain measuring heads require high, other measuring heads require lower supply currents, depending among other things on the sensor and the gases to be monitored, i.e. whether inflammable, toxic, explosive or other gases are to be detected, and in addition the very low analog signal voltages are responsible for the extremely high sensitivity to disturbing voltages which is normally a characteristic of such systems. Consequently, it has been general practice to install the individual connection lines between the individual measuring heads and the electronic evaluation system, whether they are of the 5-wire or of the 3-wire type, in the form of shielded cables. Normally, each measuring head is provided with a separate power pack feeding the sensor connected to it. Because of the many different requirements, measuring ranges, types of gases to be analyzed, and the like, one has heretofore never succeeded at arriving at a standardization in the meaning of a desirable simplification.
It has been known in connection with certain chemical sensors, i.e. such with low current consumption, to supply the measuring head with a weak supply current and to feed back a measuring signal via the same line, in which case the supply current may, for example, be in the range of maximally 4 mA, while the measuring system may be in the range of between 4 . . . 20 mA.
However, in the case of these known systems, the calibration of the sensors, which has to be effected particularly frequently, is also very troublesome and has in any case to be effected every time the sensor is exchanged, and in addition the sensor must always be connected to the one measuring head to which it is assigned.
The before-mentioned calibration may be carried out, for example, in such a manner that one service person is present at the central station, i.e. near the electronic evaluation system, while another service person, equipped with cylinders containing test gas of a given concentration, is present at the very place, i.e. the place of the respective sensor, to supply the latter with the desired test gas. The two persons at the central station and at the place of the sensor, respectively, communicate via walkie-talkie. Every time the respective sensor has been supplied with test gas in sufficient concentration, until an equilibrium has been achieved, this fact, and the type of test gas used, is then communicated to the service person waiting at the central station who then performs the calibration process on the setting potentiometers at the central station, provided no change of concentration has occurred in the meantime for some reason or other. This calibration method is lengthy and susceptible to trouble due to the fact that it has to be carried out by two people so that calibration errors are frequently encountered. Other problems encountered with the methods according to the prior art are connected with the fact that the shielded wires can be run only over a maximum length of 1500 m, due to the high current of the measuring head and the susceptibility to trouble of the weak signals of the measuring head. In addition to the high cost of installation, it is also necessary in this connection to balance the bridge current of the measuring head relative to the length of the line, and of course also relative to zero and the sensitivity.
Given the fact that the known gas detection systems do not comprise any "site electronics"--this term is being newly introduced by the present invention--one further requires, in addition to the balancing potentiometer for the bridge current in relation to the length of the line, one balancing potentiometer for zero point, one balancing potentiometer for sensitivity (measuring-head signals voltage), balancing potentiometers for the first and the second alarms, test jacks for bridge current and measuring-head signal (aging) and a separate possibility to balance the sensor or the head manually using an external measuring instrument, and all this without the possibility to combine different types of measuring components in the component carrier.
Now, it is the object of the present invention to remedy all these inconveniencies and to improve the known gas detection systems in such a manner that it is now possible, without great input of work and cost, to arrange measuring heads at any desired point, which measuring heads are capable of determining simultaneously, i.e. in parallel, the concentration of a plurality of gases and of communicating fully with the electronic evaluation system arranged at the central station, or, to say it in other words, that it is now possible, in addition to the transmission of the supply voltage to the respective measuring head, to implement bidirectional signal transmission between the measuring head and the central electronic evaluation system.